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Application of gentle annular gas veil for electrospinning of polymer solutions and melts

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A novel experimental device for elevated-temperature electrospinning of highly volatile and quickly crystallizing polymer solutions and melts was developed. The main parameters of gentle annular gas veil in electrospinning process were studied. The
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   Application of Gentle Annular Gas Veil forElectrospinning of Polymer Solutions and Melts D.M. Rein, 1  Y. Cohen, 1  A. Ronen, 1 K. Shuster, 2 E. Zussman 21 Department of Chemical Engineering, Technion-Israel Institute of Technology, Haifa, Israel 32000 2 Department of Mechanical Engineering, Technion-Israel Institute of Technology, Haifa, Israel 32000  A novel experimental device for elevated-temperatureelectrospinning of highly volatile and quickly crystalliz-ing polymer solutions and melts was developed. Themain parameters of gentle annular gas veil in electro-spinning process were studied. The influence of elec-trical conductivity of solvent on the diameter of elec-trospun fibers from ultra-high molecular weight poly-ethylene was experimentally determined.  POLYM. ENG.SCI., 49:774–782, 2009.  ª 2009 Society of Plastics Engineers INTRODUCTION Electrospun nanofibers and membranes exhibit manypotential applications, including filtration, drug delivery,sensors, protective clothing, etc [1–6]. To date, hundredsof different polymers have been successfully electrospun[7]. However, there are many potentially attractive poly-mers, such as polyolefins [4] or poly(hyaluronic acid) [8],which up to now could be electrospun only with great dif-ficulty and do not yield the stable product. Polyolefins arenot particularly amenable to electrospinning because of sol-ubility and conductivity issues. They demand manipulationunder relatively high temperatures of spinning to avoidphase separation or crystallization in polymer solutions or melts, which impedes jet formation and its subsequentelongation and causes the occurrence of jams in the die.The dielectric properties of polyolefins and their nonpolar solvents result in the need to use additives that enhance theconductivity of the spinning melts or solution [4, 5].In conventional fiber spinning, the total tensile forceremains essentially constant throughout the spun fiber,whereas in electrospinning, the tensile force decreases inthe spinning line alone. When the process is electricallydriven, thinning of the fiber occurs mostly at the nozzlearea (Taylor cone) [9], which requires the polymer solu-tion temperature and concentration to be well-controlledat this initial stage of electrospinning process [4]. The de-velopment of a suitable jet electrospinning process andformation of the desired fiber structure are controlled byseveral factors, the most important of which are the poly-mer solution concentration, conductivity, and temperature;environmental air humidity, velocity, and temperature;solvent evaporation rate and vapor concentration aroundthe Taylor cone [10–13]. The surface morphology of elec-trospun-polymer fibers is also strongly influenced by theseparameters [4, 5], [13, 14]. In cases where successfulelectrospinning of polymer solutions is difficult, due tohigh viscosity or rapid polymer crystallization, it seemsnatural to utilize other forces in addition to the electricforce in the electrospinning process, such as a flowing gasstream. Heated air jets are widely used in industry for metal and fuel spraying and, particularly, in melt blowingmanufacture of polymer fibers [15]. Unfortunately, fiber formation with sub-micron fiber diameters cannot beaccomplished only by using the mechanical gas shear/ elongation and drag forces, utilized in melt-blowing. Inthe conventional air blowing process, for fabrication of micron-scale fibers with final average diameter near 5 l m, air with initial speed of about 200 m/s is used, com-parable to the speed of an airplane [16]. For manufactur-ing of sub-micron fibers the so called ‘‘blowing assistedelectrospinning process’’ was proposed [8], [17, 18]. Inthis process, the electric force is the dominating factor,while the gas-blowing feature can assist in controllingevaporation and crystallization of the polymer solution or melt. This method was found to be well suited to controlpolymer melt crystallization. However, the high tempera-ture of the blown air instigated difficulties in controllingfiber formation from solution due to fast solvent evapora-tion rate and its effect on a rapid viscosity increase [8],[17]. Confining the capillary tip in a solvent-saturatedatmosphere could prevent premature solvent evaporation,but the tip could be blocked due to polymer crystalliza-tion that could be provoked by the cooling of air or solvent vapors spout [18]. Analysis of patent information Correspondence to : D.M. Rein; e-mail: cerycdr@tx.technion.ac.ilContract grant sponsors: Center for Absorption in Science of the Ministryof Immigrant Absorption, Committee for Planning and Budgeting of theCouncil for Higher Education (KAMEA program); Israel MOD.DOI 10.1002/pen.21273Published online in Wiley InterScience (www.interscience.wiley.com). V V C 2009 Society of Plastics Engineers POLYMER ENGINEERING AND SCIENCE—-2009  [19–23] suggests drawing a conclusion that in all hithertoinvented devices and apparatuses for the electro-blownspinning processes, the employed gas stream provides themajority of the forwarding forces in the initial stages of electrospinning of the fibers. This gas stream simultane-ously strips away the mass boundary layer along the indi-vidual fiber surface thereby, greatly increasing the diffu-sion rate of solvent from the polymer solution.The purpose of this work, in continuation of a previousstudy [4], is to develop an electrospinning device bywhich the individual effect of the parameters mentionedabove can be investigated, as well as their influence onthe structure and properties of the resultant nanofibers.The developed apparatus reduces to minimum the influ-ence of air turbulences and drag force on the spinningprocess, and allows controlling the composition of thesurrounding atmosphere near the electrospinning fibers.The device presented in this report may be less con-strained by the previously mentioned drawbacks. Themodel system used for the electrospinning experimentswas the same as reported in the earlier report: solution of ultra-high molecular weight polyethylene (UHMWPE) ina mixture of p-xylene (pX) and cyclohexanone (CH) [4]. DESCRIPTION OF NOVEL EXPERIMENTALEQUIPMENT The main idea is design and construction of a ‘‘gasveil’’ apparatus associated with the electrospinning device.Construction of this tool is illustrated in Fig. 1. As isschematically shown in Fig. 1a, the gas veil zone ( 4 ) con-sists of a hot mixture of gas and solvent vapors flowingaround the emerging polymer solution jet. The annular hot gas stream formed in the annular channel (region  1  inFig. 1) isolates the Taylor cone and the nascent solution jet from environmental influence for some distance fromthe orifice. The conditions in the gas veil zone, requiredfor stable operation, are maintained owing to the gas/ vapor mixture preparation apparatus (Fig. 1b). This appara-tus elaborates the gas (nitrogen (N 2 ) or air)/solvent vapor mixture with predefined volumetric flow rate, temperature,and solvent vapor concentration. Compressed N 2  or air controlled by pressure reducer A, maintains the needed vol-ume flow rate of the gas, which is subsequently heated inchamber B, from which it passes through the flow meter Cand liquid evaporation chamber D, with temperature con-troller at the exit (F). In chamber D, the composition of the flowing heated gas is adjusted by saturating withvapors of the required solvent and/or additional liquids, bycontinuous spraying using a metering pump (E).The annular veil of hot gas mixture flowing around theelectrospun jet allows maintenance of a predefined tem-perature of the die needle and the emerging polymer jet.This helps to avoid undesirable phase separation in thepolymer solution in the initial stage of fiber formationand consequent jamming of the die. The simplest methodto eliminate solvent evaporation is to fully saturate thesurrounding gas veil with solvent vapors, which alsoensures constant solvent composition in the polymer jet.This is especially important when using a solvent mixtureof low- and high-boiling components. It is also possibleto saturate the gas by vapors of several different liquids,which boil in different sections of the bottom tray in thesaturation chamber. During subsequent gas flowingthrough the annular die and out of it the temperature of the vapor mixture is continuously reduced, so that therelative solvent substance in vapor mixture is maintainedconstant (100% relative content). Alternatively, predefinedquantity of solvent for evaporation can be continuouslyinjected using pump E. Thus, our novel device is a multi-functional tool, which opens a lot of research possibilities,such as reducing a solvent evaporation near the die and inthe nascent jet, preventing a premature increase in solu-tion jet viscosity and polymer crystallization; maintaininga controlled temperature of nascent jet, which preventsthe premature polymer crystallization and later promotesthe drying of electrospun fibers before reaching thecollector, etc. Our novel device possesses the increasingof the charge on the solution droplet and nascent jet byusing polar vapors in the gas veil, which is important for fiber drawing in the electrospinning process [10]. The useof specific vapors in the gas veil, which can condense onthe solution jet, is another feature that may be utilized toeffect new surface morphologies during the electrospin-ning process [13, 14], [24]. These components may beadded regardless of their miscibility with the main polymer solvent or possibility to cause the coagulation of thedissolved polymer. If the condensed liquid layer on thefiber surface has sufficient conductivity, it may also promotecharge accumulation on the electrospun fiber surface. Our novel device also allows controlling or even eliminating theatmospheric moisture influence on the electrospinningprocess [13]. INVESTIGATION OF NOVEL EXPERIMENTALEQUIPMENT  Aerodynamic and Thermal Properties of a Gentle Annular Gas Veil Most investigations of jet aerodynamics are devoted tocircular jets [25–27]. An annular jet has significantly dif-ferent flow characteristics. The presence of an inner wallinduces the occurrence of a specific recirculation zoneclose to the die orifice [28]. The dominant characteristicof this zone is the occurrence of an area where the flow istraveling in the opposite direction relative to the jet [29].This strongly influences the temperature field in this area.In all recent experimental studies on annular jets, the ve-locity, temperature and vapor fields were investigated inareas of fully developed flow (which begins approxi-mately 10 hydraulic diameters from the nozzle), signifi- DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2009 775  cantly removed from the recirculation zone [30, 31]. Thebehavior of undeveloped, weak (gentle) annular jets, andtheir recirculation zone is of main interest for this appara-tus because of its significant influence on the gas veil inour process. Numerical simulations of this zone usingpopular   k  - e  and  k  - x  models did not provide acceptable FIG. 1. (a) Construction of the spinning die with annular gas veil:  1  —die gas/vapor mixture annular chan-nel;  2  —die needle;  3  —electric heating spiral and Teflon 1 thermal isolating coating on the needle;  4  —gas veilzone around Taylor cone and initial part of polymer solution jet;  d  o  —external diameter of annular gas channel, d  i  —internal diameter of gas channel, L—gas channel length. (b) Apparatus for preparation of the hot gas/vapor mixture: (A) compressed gas pressure reducer; (B) gas heating chamber with internal electric heating spiral,(C) gas flow meter; (D) gas saturation chamber incorporating: for liquids evaporation with spray nozzle (top)and a bottom tray divided into sections for a different liquids, heated by a silicon oil bath, (E) meter pump for the adjusted supply of liquids subjected to evaporation; (F) gas/vapor mixture temperature controller. 776 POLYMER ENGINEERING AND SCIENCE—-2009 DOI 10.1002/pen  predictions, because of principally anisotropy turbulencecharacteristics of the flow regime [28, 29].We undertook the experimental temperature mappingof the gas veil zone with a fine iron-constantan thermo-couple of the exposed junction type. The junction diame-ter was 0.7 mm, and the thermocouple was pointed upinto the gas flow. This arrangement minimized the dis-turbance of the gas flow. The thermocouple was mountedon a traverse unit, which allowed its accurate motion inthe flowing gas. The main construction parameters of our device are listed in Table 1 (see also Fig. 1a).As was previously mentioned, the main purpose of gasveil zone is to maintain the temperature of the die needleand polymer solution near the die orifice and to minimizesolvent evaporation from the nascent polymer jet. Thenecessary temperature in the gas veil should be higher than a critical temperature of phase transition, which hindersthe electrospinning process. In this case, it was deter-mined to be about 110 8 C, the cloud point temperature of the UHMWPE solution at the experimental concentration.As the initial jet temperature is increased, the sensitivityof the spinning process to the solvent vapor content in theimmediate vicinity of a die orifice assumes a determiningimportance. In particular in our case, this occurs when theinitial polymer jet temperature exceeds 138 8 C (the boilingtemperature of the most volatile solvent component (pX)).The typical results of die temperature tests during flowof the annular gas veil are shown in Fig. 2. It was foundthat gas flow in the immediate vicinity of a nozzle orificehas three specific regimes. (A) At relatively high gas vol-ume flow rates (more than about 250 standard L/min): aninner annular gas wall begins to appear, the recirculationzone arises, and temperature distribution in the gas veilzone begins to be highly irregular (Fig. 2a). The sur-rounding cold air is sucked into the annular gas veil lead-ing to undesirable cooling of die orifice. Stable electro-spining is not possible under these conditions. (B) Atintermediate gas volume flow rates (about 50 to 250standard L/min): the inner annular gas wall is not welldeveloped and temperature distribution in the gas veilzone is relatively uniform (Fig. 2b). (C) At small volumeflow rates (less than about 50 standard L/min): the inner annular gas wall is absent (the annular gas veil is fullyflooded) and influence of the heated spout is local. Theuseful gas veil zone converges (Fig. 2c). (D) In thisregime, complementary heating of the die needle usingthe built-in electric heater (Fig. 2d), allows formation of well-developed gas veil zone and reduction of the temper-ature gradient within it. At greater flow rates of gas theinfluence of nozzle heating on the temperature field near the die orifice become negligible.The temperature distribution in the veil zone at differ-ent gas volume flow rates ( G , standard L/min), may beconveniently exhibited by the dependence of the relativeexcess temperature  y  /  y e  on the nondimensional axial dis-tance  Yd  h 2 1 ( q 1  /  q ) 2 1/2 [31]. Here,  y  ¼  T  2 T  a  is localexcess gas temperature above ambient, ( 8 C);  y e  ¼  T  e 2 T  a is the excess gas temperature above the ambient one atthe nozzle exit.  T  ,  T  a , and  T  e  are the local, ambient, andinitial gas veil temperatures, respectively, ( 8 C). Y—axialposition, (mm);  d  h  ¼  d  0 2 d  i  —hydraulic diameter of annu-lar orifice, (mm);  q 1 ,  q  —air density at ambient condi-tions and at a local point in the gas stream, respectively(kg/m 3 ).Figure 3 shows the temperature distribution along theveil centerline and along a line with radial coordinate  r  i ¼  d  i  /2 (see Fig. 2), which is taken as a lateral boundaryof the gas veil zone. Figure 3a shows that at flow re-gime A the centerline and gas veil zone lateral boundarytemperature profiles are strictly dependent on the gasvolumetric flow rate. Figure 3b illustrates that at flowregime B both the centerline and gas veil zone lateralboundary temperature profiles are independent of thegas flow rate and its initial temperature. Figure 3cshows that at flow regime C the temperature field in thegas veil is similar to that of circular-shape jets [25–27],but the influence of complementary nozzle heating ispronounced.In terms of the practical range of gas/vapor veil initialtemperatures, from about 125 to 190 8 C needed for our spinning solutions, the average relative excess temperaturein the gas veil zone must be more than about 0.5 (thisapproximately corresponds to a temperature of 110 8 C).Comparison of regimes A, B and C (see Fig. 3) showsthat only gentle regimes B and C have an acceptable av-erage temperature in the gas veil. The relative excess tem-perature difference between the centerline and lateralboundary of the gas veil (with taken diameter of 1 d  i ) is inthe acceptable range at a distance of about 2 d  h  from thedie orifice. In regime C with additional die needle heatingthe acceptable range of gas veil zone may be prolongedup to 4 d  h  from the die orifice (Fig. 3c).It should be noted that specific measurements haveshown that the gentle annular gas veil does not signifi-cantly affect the spreading of fiber on the collector sur-face during electrospinning, up to gas flow rates of about300 standard L/min. In fact, fiber spreading may be evensomewhat decreased, in case of tangential air injectioninto the cylindrical chamber, built specially around thedie [32]. In our device, the need for additional chambersis absent. The gentle annular gas spout creates a control-lable gas veil with diameter about of 12 mm and lengthup to 50 mm (from the die orifice along its axis) at practi-cally acceptable volumetric gas flow rates in range of about 15–50 standard L/min and veil temperatures inrange of about 125 to 190 8 C. TABLE 1. Construction parameters of the device.Parameter ValueNeedle inner diameter (mm) 0.15External diameter of annular gas channel—  d  0  (mm) 17Internal diameter of annular gas channel—  d  i  (mm) 11Annular gas channel length—   L  (mm) 20 DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2009 777   Electrospinning of UHMWPE Solutions using Gentle Annular Gas Veil All materials that were used for electrospinning experi-ments are the same as reported in details earlier [4] andare listed in Table 2. The polymer was dissolved in differ-ent mixtures of pX and CH at weight ratios, pX/CH (w/ w), of 1:1, 1:3, and 1:11. All mixtures were heated toabout 135 8 C before addition of UHMWPE powder andsubsequently stirred at the same temperature for 1 h.The polymer solution concentration was 0.05 wt%. of UHMWPE in all experiments. The main electrospinningparameters were the same, as was described by us previ-ously [4], and are listed in Table 3. Parameters of the FIG. 2. The temperature field within the annular gas veil. (a) temperature distribution near the die orifice atflow regime A (gas volume flow rate 300 standard L/min, initial gas temperature 145 8 C); (b) at flow regimeB (gas volume flow rate 80 standard L/min, initial veil temperature 170 8 C); (c) at flow regime C (gas volumeflow rate 25 standard L/min, initial gas temperature 125 8 C, without nozzle heating); (d) at flow regime Cwith additional nozzle heating (gas volume flow rate 25 standard L/min, initial veil temperature 125 8 C, noz-zle heating temperature 130 8 C). 778 POLYMER ENGINEERING AND SCIENCE—-2009 DOI 10.1002/pen
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